Mechanical operations such as piercing, punching, shape forming and resistance welding are common to many industrial fabrication processes. In many instances, it is desirable or necessary to perform such operations on a relatively thin section workpiece which is already finished to proper size and material specification; for example, it may be necessary to pierce or punch a hole or group of holes in a metal bracket which is stamped, finished and heat treated prior to the piercing or punching operation. Under these circumstances, it is desirable to avoid any significant deformation or reshaping of the workpiece during the piercing or punching operation. This can be achieved through the use of a so-called "self-equalizing" fabrication device or machine; i.e., a device or machine which engages and applies equal forces to the opposite sides of the workpiece during the mechanical operation so that undesirable inelastic deformation is avoided.
Prior art devices which are capable of accomplishing mechanical operations is self-equalized fashion are disclose in U.S. Pat. No. 3,008,032 to Wolfbauer, Jr. and U.S. Pat. No. 3,396,260 to Waltonen. In both of these patents, the mechanical operation is resistance welding and the term "mechanical operation" is used herein to encompass not only resistance welding but piercing, punching, pressing, rivoting, stamping and shaping operations.
In both the Wolfbauer and Waltonen patents the apparatus comprises a base which may be bolted or otherwise secured to a support surface, a yoke member which is bidirectionally displaceable relative to the base along a work axis and which comprises or supports an element which engages and supports one side of a workpiece during a mechanical operation. The Wolfbauer and Waltonen devices further comprise an hydraulically operated piston which is displaceable along the work axis relative to the yoke and which comprises or supports a second element which contacts the other side of the workpiece during the work stroke. Due to the arrangement of the yoke and piston, the application of hydraulic pressure to the piston causes the piston carried work element to advance toward the front side of the workpiece as the yoke advances toward the back of the workpiece to effectively clamp the workpiece in a self-equalized fashion which avoids or prevents the deformation which might result from applying mechanical forces to only one side of the workpiece.
In both of the prior art patents mentioned above, the piston and cylinder arrangement are substantially wholly external to the yoke thus giving rise to the necessity for numerous components and mechanical connections which accommodate the relative displacement or travel of the various parts. In addition, the devices tend to be relatively complex and difficult to mount in varying orientations.